The invention presented here concerns a process and device for manufacturing blisters which have high barrier properties using a metal-plastic composite foil, in particular an aluminum-plastic composite foil which is claimed in the clamping facility of a device for making the blisters and such that the clamped foil cannot move.
In the conventional deep drawing process used up to now it is a disadvantage that the deep drawing ratio must be made to suit the critical range of radius i.e. reduced as necessary, because of the non-uniform stressing of the metal-plastic composite foil during deep drawing.
What is known, for example from the German Pat. No. 2 231 980, it is a device for producing cup-shaped recesses in thin aluminum-plastic composite strip using the plunger of a die and a clamping facility which holds the strip firmly on the die. The shaping surface of the plunger firmly holds in place a thin layer of material which has a low coefficient of friction. The purpose behind such measures is to prevent cracks from forming in the aluminum-plastic composite during the deep drawing operation and to prevent the composite from being subjected to relatively large frictional forces during deep drawing. The frictional forces which arise during the deep drawing operation do not depend only on the coating of the plunger but also on the coefficient of friction of the metal-plastic foil. In the case of foodstuffs and pharmaceuticals, special lubricants are often not permitted on the inside of the blister. Since this is so, small changes in the surface of the metal-plastic composite foil lead to variations in deep drawability or to interruptions in production.
Furthermore, for example in the German Pat. No. 2 539 354, a process and device for manufacturing light weight containers from foils of metal, plastic or preferably from a metal-plastic composite foil are described, whereby the foil is held firmly at the edge of an opening and stretched into a cup shape in the opening by means of a plunger, and where the head of the plunger first comes to rest in the inner part of the freely spanned part of the foil, stretches this to a pre-form shape with slightly inclined sidewalls and then by advancing the stroke of the plunger one, or several edges of the shoulder of the plunger one after another, come to rest against the inclined sidewall and broaden and deepen the pre-form. This process causes the blister to have a stepped sidewall or a very large radius between sidewall and base, which is not desirable.
In deep drawing metal-plastic composites such as aluminum-plastic composites the foil or strip material, in the above mentioned known processes and devices, is heavily stressed especially in the region of the radii or in the zones of the numerous shoulder edges of the plunger between the base and the sidewall, as a result of which the deep drawing ratio cannot be increased further. Furthermore, as far as the coating of the plunger with a low friction material as in the German Pat. No. 2 321 980 is concerned, it should be pointed out that such a coating is subject to relatively large amount of wear and therefore it is not possible to have continuous, uninterrupted operation with such a plunger.
Another disadvantage of the above mentioned processes and devices for carrying them out is that the basal areas of a semi-rigid blister during deep drawing are not, or only partly, subjected to elongation or pre-stretching, and therefore up to now the frequently, relatively large basal areas of the blister were subjected only to partial stretching, and consequently only relatively small deep drawing ratios could be achieved.
The object of the invention presented here is to develop a process and device for producing blisters from a metal-plastic composite which can be deep drawn, in particular from an aluminum-plastic composite, by means of which a larger deep drawing ratio and/or radii can be achieved along with steeper sidewalls in the blisters and thus larger volume blisters than was possible up to now, whereby it is also possible to achieve better shaping or matching of the blister to the contours of the contents, and whereby the disadvantages described in connection with the previously mentioned processes and devices are avoided.